Weighing devices are generally designated as scales or balances. Scales are devices which measure displacement in response to a load being weighed, while balances are devices wherein a quantifiable (i.e., known) opposing force is utilized to counter, or balance, a load without occurrence of substantial mechanical displacement.
Scales are usually low-resolution devices which employ a spring of some nature which distends in accord with Hooke's law. By measuring the calibrated spring displacement, the weight of the applied load may be determined. One example of an advanced scale utilizes a parallelogram suspension which resolves the normal force to be measured from force couples, such as off-center loading torques, that are not of interest. Such suspension is also known as Roberval's mechanism (see U.S. Pat. No. 4,582,152, which though characterized as a balance is actually a scale).
Heretofore known balances include mechanical equal-length beams with a center fulcrum which compares the mass of an unknown load on one side of the fulcrum to a known mass on the other. When the two masses are balanced across the fulcrum, the unknown mass is equal to the known mass. While capable of excellent resolution, mechanical balances require tedious fine adjustments.
Electronic balances often utilize a variable electromagnetic opposing force to counter the unknown load. A position sensor, capacitance or optical typically, measures incipient displacement from a null position and varies the electromagnetic force to balance the unknown load while maintaining the null position. Thus, the electrical current, calibrated relative to known masses, may be measured and, as a function of the balance load mass, provides an output indicative of the mass of the load (see, for example, U.S. Pat. Nos. 3,680,650 and 4,034,819).
One type of basic electronic balance, known as a “direct load” balance or a “direct loader” (as shown in U.S. Pat. No. 3,680,650), utilizes an armature or post suspended for vertical movement and which carries a pan and the moveable plate of a capacitor null sensor. A fixed plate or plates complete the capacitor such that displacement of the moveable plate in response to a load on the pan modulates an electronic circuit to produce current flow through a coil also carried on the armature. The coil is disposed within a permanent magnet to form a speaker coil structure. The field generated by the coil current in conjunction with the field of the permanent magnet produces a force applied to balance a load on the pan. Accordingly, the current produced to balance a given load is an accurate function of the applied load. This type of balance, while simple and relatively accurate, tends to be somewhat capacity limited, primarily due to heat developed by the current flowing through the coil and the electrical resistance of the coil. Such heat increases as the square of the current. Eddy currents in the magnet also contribute to heat. The magnet, which looses field strength as its temperature increases, must of course be adjacent the coil and is readily heated by the coil. Also, such heat can be conducted to the mechanical structure of the balance where component expansion contributes to weighing errors.
Higher load capacity electronic balances are known and or utilized (see U.S. Pat. No. 4,109,738), wherein the coil assembly is connected to the pan carrying armature structure by a lever structure which provides for mechanical amplification of the force generated by the coil and magnet arrangement. The current required to support a given applied load is thus diminished. Such balances, however, involve substantial mechanical complications, require additional fragile suspension and introduce further sources of error in lever structures over “direct loader” type balances.
Another approach heretofore known and/or utilized for addressing the deleterious effects of heat resulting from current passing through the coil of electronic lever balances utilizes mechanical counterbalancing (or offset) of the lever by mass on the coil side of the lever to about one-half of the balance's rated capacity (see U.S. Pat. No. 3,955,638, utilizing an optical null sensor). Thus the null sensor initially causes coil current to flow in a direction to produce a resultant force which is additive to the pan load. Only when the counter balance mass is exceeded by the pan load does the coil current produce a force opposing the pan load. In such case the heat generated by the coil current is one quarter of that produced by a balance that fully counterbalances the pan load with current flowing in a single direction. Such balances, while effectively reducing heat in the system, introduce sources of weighing error inherent in lever and mass structures and could still be further improved to allow for wider capacity range and more compact design.